Height tailoring of interfacing projections

ABSTRACT

A method of joining a first component to a second component, the method comprising forming an array of projections extending from a bond surface of the first component, the projections having a plurality of different profiles; and embedding the array of projections in the second component formed of a plurality of laminate plies, wherein each projection profile is adapted to best transfer load into a respective one of the laminate plies. The resultant joint is able to transfer load more progressively between the two components leading to improved tensile strength.

RELATED APPLICATIONS

The present application is based on, and claims priority from, BritishApplication Number GB0905134.3, filed Mar. 25, 2009, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method of joining a first componentto a second component, and to a joint so formed.

BACKGROUND OF THE INVENTION

Joining between metallic or thermoplastic and composite components iscurrently approached in a number of ways, each with its own limitations.

The use of fasteners is commonplace but tends to result in de-laminationaround fastener holes. Fastener holes are often difficult to drill incomposites and significant reinforcement around fastener holes may berequired, leading to increased weight. Fastened joints tend to beparticularly weak in the pull-off direction (that is, the direction ofaxial load through the fastener). As such, fastened joints are not wellsuited to many aerospace applications.

Adhesive bonds are an increasingly common means of joining metalliccomponents to composite laminates, however these perform poorly in peel,tension and cleavage, and tend to fail with little or no warning. Theirweakness in peel and in tension makes bonded joints similarly limited intheir application within conventional aerospace structures. Anymitigation for the poor performance in peel or tension tends to resultin large bond surface areas, with the associated weight penalties thatgo with this.

WO 2004/028731 A1 describes a method by which surface features aregenerated by using a ‘power-beam’ such as an electron beam, in order to‘flick-up’ surface material on a metallic component to sculpt protrudingfeatures that are intended to increase bond surface area and improvebond strength when incorporated into the matrix of a co-cured laminate.The displacement of surface material to create the protruding featuresis likely to generate crack initiators that will adversely affect thefatigue life of the component. Also, it is difficult to optimise theprofile and shape of the surface features.

WO 2008/110835 A1 describes a method by which surface features are“grown” on a bond surface of a component in a series of layers by anadditive fabrication process. The profile and shape of the surfacefeatures can be controlled so as to tailor the performance of the joint,particularly in tension and peel. Each surface feature may have apointed tip such that the surface features may easily be embedded into aseries of laminate plies draped successively over the bond surface.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of joining a firstcomponent to a second component, the method comprising forming an arrayof projections extending from a bond surface of the first component, theprojections having a plurality of different profiles; and embedding thearray of projections in the second component formed of a plurality oflaminate plies, wherein each projection profile is adapted to besttransfer load into a respective one of the laminate plies.

A second aspect of the invention provides a joint between a firstcomponent and a second component, the first component having an array ofprojections formed extending from a bond surface thereof, theprojections having a plurality of different profiles, and the secondcomponent comprising a plurality of laminate plies, wherein the array ofprojections are embedded in the second component, and wherein eachprojection profile is adapted to best transfer load into a respectiveone of the laminate plies.

The joint is advantageous in that axial load in the projections is moreprogressively transferred into the laminate of the second component.This improves the “pull-off” tensile strength of the joint in thedirection normal to the bond surface. In prior art joints where thearray of identical projections are closely packed, the dominant failuremode in the pull-off tensile direction can be a net section failure ofthe laminate (i.e. de-lamination of the plies) in a stress concentrationregion around the projections. By varying the profiles of theprojections in the array, the load can be transferred into a greaternumber of the laminate plies of the second component, so reducing thesestress concentrations and improving the tensile strength of the joint.

In one embodiment, the projection profiles are varied by changing theheight of their overhanging edge(s) with respect to the bond surfacesuch that the overhanging edges engage with different plies in thestack. The region around the overhanging edges transfers the majority ofthe “pull-off” load from the first component into the plies of thesecond component. By changing the height of these overhanging edges, theloads transferred into individual plies may be controlled, and so thetensile strength of the joint can be improved.

In another embodiment, the projection profiles are varied by changingthe overall height of the projections with respect to the bond surfacesuch that the projections engage with different plies in the stack. Bychanging the height of the projections, the loads transferred intoindividual plies may be controlled, and so the tensile strength of thejoint can be improved.

In a preferred embodiment, the projection profiles are scalable only inthe axial direction of the projections and so the projections are all ofsimilar average width dimension. Therefore, the projections in the arrayhave variable aspect ratio (i.e. the ratio of the projection height toits base area). The aspect ratio influences the stiffness of theprojection and typically varies between approximately 2 and 6. In thisway, the projection profiles can be varied by changing both the heightof the overhanging edge(s), and the overall height, of the projections.

The projections may be arranged such that some adjacent projections havethe same, or different, profiles. The projections may be arranged ingroups including projections of different profiles such that the groupsform a repeating profile pattern in the array.

Each projection may be rotationally symmetric about its centreline.Preferably, each projection has a frustoconical base, a conical tip andan inverted frustoconical overhang between the base and the tip.Regardless of the rotational symmetry of the projections, theypreferably have a pointed tip to improve the ease of embedding theprojections into the second component, and an overhang to enhance thepull-off (tensile) strength of the joint. The projections may have alinear or curved centreline, which may be normal or form an obliqueangle at its intersection with the bond surface.

Preferably, the projections are grown on the bond surface in a series oflayers, each layer being grown by directing energy and/or material tothe bond surface. Suitable additive fabrication techniques may be a“powder bed” process (in which a series of layers of powder aredeposited on the bond region and selected parts of each bed are fused bya power beam) or “powder feed” process (in which a directed stream ofpowder is deposited on selected parts of the bond region and then fusedby a power beam, such as a laser or electron beam). Alternatively theprojections may be formed by friction welding a set of projections ontothe bond surface. Yet further, the projections may be formed by fuseddeposition modelling (in which hot plastic is extruded through anozzle).

The first component may be formed from a metallic material (such asTitanium or stainless steel); a thermoplastic material such aspolyetheretherketone (PEEK); or any other suitable material. The secondcomponent may include fibre reinforced composite laminate plies.

The projections of the first component are preferably embedded into thesecond component by draping successive laminate plies which will formthe second component over the first component. The draping may beperformed manually or a computer controlled tape laying machine may beused. A soft roller may be required to adequately embed the projectionsof the first component in the appropriate plies of the second component.The first component may be set in a mould tool over which the layers aredraped. Alternatively, the first component may be rolled, or otherwisemoved, over one or more of the plies so as to embed the projections inthe plies.

Each laminate ply of the second component may be laid up as dry fibreplies to which resin is infused after the projections are embedded.Alternatively, the fibre plies may be pre-impregnated with the resin, aso-called “pre-preg”, in which the projections are embedded. Afterembedding the projections in the plies and infusing resin, if necessary,the fibre reinforced composite plies may need to be cured. The compositeplies may be, for example, carbon fibre reinforced plastic (CFRP), glassfibre reinforced plastic (GFRP), or Aramids such as Kevlar.

Preferably, the second component is co-cured with the first componentafter the projections of the first component are embedded in the secondcomponent. Each ply of the second component may be cured separately onthe first component, or curing may be performed after a batch, or all,of the plies of the second component have been laid up on the firstcomponent.

The projections may be formed from the same material as the firstcomponent, or they may be formed from a different material.

The joint may be used to join structural components, for instance in anaerospace application. For instance the joint may be used to join areinforcing plate, floating rib foot, or stringer to a panel such as awing or fuselage cover.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a cross section view of inter-laminar failure in aprior art hybrid joint having an array of identical interfacingprojections;

FIGS. 2 a and 2 b illustrates partial cross section views of the hybridjoint of FIG. 1;

FIG. 3 illustrates a cross section view of a hybrid joint according to afirst embodiment of the present invention having an array of interfacingprojections of different profiles;

FIGS. 4 a and 4 b illustrate partial cross section views of the hybridjoint of FIG. 3;

FIG. 5 illustrates a cross section view of a hybrid joint according to asecond embodiment of the present invention;

FIG. 6 illustrates a cross section view of a floating rib foot beingintegrated into a mould tool;

FIG. 7 illustrates a composite lay-up on the mould tool of FIG. 6;

FIG. 8 illustrates a perspective view of the floating rib footconnecting a rib to a cover;

FIG. 9 illustrates a schematic view of a powder bed fabrication system;and

FIG. 10 illustrates a schematic view of a powder feed fabricationsystem.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1 shows a prior art joint known as a Hybrid Penetrative databaseReinforcement (HYPER) joint between a metallic or thermoplasticcomponent 100 having an array of projections 101,101 . . . and alaminate composite component 200. The array of projections 101,101 . . .are formed on a bond surface 102 of the metallic or thermoplasticcomponent 100 and are embedded in several laminate plies 201 of thecomposite component 200. The embedded projections 101,101 . . .significantly enhance the strength of the joint when it is subjected totensile, peel or cleavage loads.

Each projection 101 has a conical tip, the frusto-conical base, and aninverted frusto-conical overhang. The overhang has an undercut edgewhich is inclined and faces towards the bond surface 102. The conicaltip and the inverted frusto-conical overhang together form a “head” ofthe projection 101.

The prior art joint shown in FIG. 1 suffers the problem that since eachprojection 101 is identical and the projections in the array are closelypacked, the joint can fail when subjected to tensile load “L” in thepull-off direction (i.e. perpendicular to the bond surface 102) by a netsection failure of the laminate component 200. The laminate component200 fails by de-lamination of plies 201 in a region around the heads ofthe projections 101,101 . . . FIG. 1 shows the failed joint. Whensubjected to load L in the pull-off direction, the prior art joint shownin FIG. 1 has stress concentration regions where the projections 5,5 . .. best mechanically engage with the plies 201 of the composite component200. This is typically in the region around the projection heads due tothe overhanging edge beneath the maximum head diameter.

FIG. 2 a shows a partial cross section view of the HYPER joint of FIG.1, taken through the plane A-A containing the ply 201 positioned at themaximum head diameter of the projections 101,101 . . . . The penetrationarea (white) of each projection head is “p” and the ply area prior topenetration by the projections is “A”. FIG. 2 b shows a partial view ofthe HYPER joint of FIG. 1, showing only the ply 201 positioned at themaximum head diameter of the projections 101,101 . . . . The ringsaround each projection 101,101 . . . show the stress concentrationregions in the composite component 200.

FIG. 3 shows a cross section view of a portion of a HYPER jointaccording to an embodiment of the present invention. An array ofprojections 5,5 . . . is formed extending from a bond surface 6 of ametallic floating rib foot component 1. Each projection 5 also has aconical tip, the frusto-conical base, and an inverted frusto-conicaloverhang. The overhang forms an overhanging undercut edge which isinclined and faces towards the bond surface 6. The conical tip and theinverted frusto-conical overhang together form a “head” of theprojection.

Figure depicts the array of projections 5,5 . . . that includes aplurality of first projections (e.g., the shorter projections) and aplurality of second projections (e.g., the taller projections), wherethe first projections have first and second frusto-conical sectionsarranged back-to-back (e.g., referring to the two-dimensions of FIG. 3,respectively (i) the triangular portion of element 5 and (ii) thetrapezoidal portion sharing a line thereof with the triangular portionof element 5), and the second projections have third and fourthfrusto-conical sections arranged back-to-back (e.g., referring to thetwo-dimensions of FIG. 3, respectively (i) the triangular portion ofelement 5 and (ii) the trapezoidal portion sharing a line thereof withthe triangular portion of element 5). As can be seen from FIG. 3, aheight of a first area of adjacency between the first and secondfrusto-conical sections above component 1 is different than a height ofa second area of adjacency between the third and fourth frusto-conicalsections above component 1.

The projections 5,5 . . . are embedded in several laminate plies of acomposite wing cover component 18. Unlike the prior art hybrid jointshown in FIG. 1, the hybrid joint shown in FIG. 3 has projections 5 ofvarying height Z from the bond surface 6. The projections 5,5 . . .penetrate through differing numbers of plies in the wing cover 18depending on their height Z. In FIG. 3, two projection profiles areshown, one being taller than the other.

It has been found that varying the height of the projections, and inparticular the height of the overhanging edges, increases the strengthof the HYPER joint in the pull-off direction, without markedly affectingthe performance of the joint under other load conditions, e.g. shear.The joint in accordance with the present invention reduces stressconcentrations in the wing cover 18 by increasing the number of plies inthe wing cover 18 which best mechanically engage with the projections5,5 . . . , and increasing the un-penetrated area (by the projections)of each of the plies involved in this mechanical engagement.

FIG. 4 a shows a partial cross section view of the HYPER joint of FIG.3, taken through the plane B-B containing the ply positioned at themaximum head diameter of the taller projections. This ply also containsthe tip of the shorter projections. The penetration area (white) of eachprojection head is either “p” for the taller projections, or “t” (wheret=0.25p) for the shorter projections. The ply area prior to penetrationby the projections is “A”. FIG. 4 b shows a partial view of the HYPERjoint of FIG. 3, showing only the ply positioned at the maximum headdiameter of the taller projections. The rings around each projection 5,5. . . show the stress concentration regions in the wing cover 18.

Comparing FIGS. 2 a/2 b and 4 a/4 b, it has been found that the stressin the ply shown in FIGS. 4 a/4 b is significantly lower than the stressin the ply shown in FIGS. 2 a/2 b. Consider the case where eachprojection only transfers load into the ply positioned at the maximumhead diameter of the projection. If the load transferred at eachprojection is “F”, then:

FIG. 2a/2b FIG. 4a/4b Load into ply = 9F 5F Ply load transfer area =A-9p A-6p Stress in ply = 9F/(A-9p) 5F/(A-6p)

The stress in the ply shown in FIGS. 4 a/4 b is reduced when comparedwith that of FIGS. 2 a/2 b partly due to the reduced load beingtransferred into the ply, and partly due to the increased ply loadtransfer area (i.e. the area of the ply that can transfer load to anadjacent ply), as a result of the multiple projection height profiles.

It is to be noted that the projections 5,5 . . . of FIG. 3 are each ofidentical width dimension and so the varying height Z affects the aspectratio (i.e. the ratio of the projection height to its base area). Theaspect ratio influences the stiffness of the projection and typicallyvaries between approximately 2 and 6.

In the portion of the array shown in FIG. 3, the projections 5,5 . . .are of two different heights Z and projections of each height arestaggered alternately across the array. However, the projections in thearray may have more than two different heights and projections ofdifferent heights may be arranged in the array so as to form a regularor irregular pattern of virtually any form. For example, in a line ofthe array, a pair of projections having a first height may be followedby a pair of projections of a second height and then a second pair ofprojections of the first height, and so on. Alternatively, whereprojections of three or more heights are provided, a line of the arraymay have projections of gradually increasing and then decreasing heightso as to form alternating peaks and troughs of projection tips. Ofcourse, the array is two-dimensional and so these patterns can berepeated in both dimensions of the array. Some or all of the projectionsmay have more than one overhanging edge. Numerous other arrangements ofprojections of differing height will be appreciated by those skilled inthe art.

FIG. 5 shows a cross section through an alternative HYPER joint inaccordance with the present invention, which provides a stress optimisedsolution. The joint comprises a metallic or thermoplastic component 500having an array of projections 501 a, 501 b formed on a bond surface 502thereof. The projections 501 a, 501 b are embedded in several laminateplies 601 of a composite component 600. The projections comprise a setof projections 501 a, 501 a . . . having a first projection profile, anda second set of projections 501 b, 501 b . . . having a secondprojection profile. The projections 501 a each have a series offrusto-cones arranged back-to-back to form three regions of increaseddiameter. The projections 501 b each have a series of frusto-conesarranged back-to-back to form two regions of increased diameter. Beneatheach region of increased diameter is an overhang portion having anoverhanging edge. The projections 501 a, 501 b each have a conical tip.

The projections 501 a,501 b are arranged on the bond surface in athree-dimensional array of alternating profiles 501 a, 501 b, 501 a,etc. Load is transferred from the projections into the plies 601 at theregions of increased diameter. The circled areas show the stressconcentration regions in the joint. Where a plurality of regions ofincreased diameter are provided on each projection, arranged to transferload into the surrounding plies, one of these regions of increaseddiameter will be responsible for transferring the majority of the load.This will be the case even when the regions of increased diameter aresimilar in profile. By arranging the projections 501 a, 501 b as shownin FIG. 5, the stress in each ply 601 can be managed by reducing theload transferred into each ply, and by increasing the ply load transferarea (i.e. the area of the ply that can transfer load to an adjacentply).

The increased number of overhanging edges in the HYPER joint shown inFIG. 5 in comparison with the HYPER joint of FIGS. 4 a and 4 b reducesthe load transferred into each ply, and so reduces yet further thestress in each ply so improving the tensile strength of the joint.However, the projections of the embodiment shown in FIG. 5 are morecomplex to manufacture.

It is not essential that the overall height Z of the projections in thearray is variable where the projections have overhanging edges ofdiffering heights. However, when tailoring the projections for improvedtensile strength performance of the joint, it is preferable that all ofthe projections in the array are scalable in the axial direction so thatthe performance of the joint under other load conditions (e.g. shear) isnot impaired. Accordingly, the height Z of the projections preferablyvaries with the height of the overhanging edges.

A method of forming the hybrid joint shown in FIG. 3 will now bedescribed in detail. FIG. 6 shows a cross section of the whole metallicfloating rib foot 1. The floating rib foot 1 comprises a downwardlyextending web portion 3 and a pair of outwardly extending flanges 2. Theweb portion 3 has a pair of fastener holes 4 (one of which is shown inFIG. 6). The array of projections 5,5 . . . extend upwardly from a bondsurface 6 of the flanges 2. The projections 5 are distributed around theperiphery of the flanges 2 and surround a central region with noprojections.

The floating rib foot 1 is integrated into a mould tool 10. The mouldtool 10 has a mould surface 12 with a recess 11 which receives theflanges 2 as shown in FIG. 7. Web portion 3 extends into a channelbetween a pair of plates 13, 14, and is secured in place by a fastener17 passing through a pair of holes 15, 16 in the plates 13, 14 as shownin FIG. 7. In the example of FIG. 7 only one fastener 17 is shown, butin alternative arrangements two or more fasteners may be used to securethe floating rib foot to the mould tool. In the case where two fastenersare used, then they may be passed through the holes 4 in the web portion3.

After the floating rib foot 1 has been integrated into the mould tool10, a composite lay-up 18 is laid onto the mould tool. The compositelay-up 18 comprises a series of plies of uni-axial carbon fibre,pre-impregnated with uncured epoxy resin. Each ply is conventionallyknown as a “prepreg”. The initial prepregs are penetrated by theprojections 5 as shown in FIG. 7.

After the lay-up 18 has been formed as shown in FIG. 7, it is cured andconsolidated by a so-called “vacuum bagging” process. That is, thelay-up is covered by a vacuum membrane (and optionally various otherlayers such as a breather layer or peel ply); the vacuum membrane isevacuated to apply consolidation pressure and extract moisture andvolatiles; and the lay-up is heated (optionally in an autoclave) to curethe epoxy resin matrix. As the epoxy resin matrix melts prior to cure,it flows into intimate contact with the projections 5. The projections 5mechanically engage with the matrix, while also increasing the surfacearea of the bond.

The components are then removed from the mould and assembled withvarious other wing box components as shown in FIG. 8. In this examplethe cured lay-up 18 is a wing cover, and the floating rib foot 1 securesa rib to the wing cover 18. The rib comprises a rib web 20 and a fixedrib foot 21 extending downwardly from the rib web 20. Fasteners (notshown) are passed through the fastening holes 4 in the web portion 3 ofthe floating rib foot 1 to secure the floating rib foot 1 to the fixedrib foot 20.

Each projection 5 is grown on the bond surface 6 in a series of layersby an additive manufacturing process: either a powder bed process asshown in FIG. 9, or a powder feed process as shown in FIG. 10.

In the powder bed process shown in FIG. 9, the array of projections isformed by scanning a laser head laterally across a powder bed anddirecting the laser to selected parts of the powder bed. Morespecifically, the system comprises a pair of feed containers 30, 31containing powdered metallic material such as powdered Titanium. Aroller 32 picks up powder from one of the feed containers (in theexample of FIG. 9, the roller 32 is picking up powder from the righthand feed container) and rolls a continuous bed of powder over a supportmember 33. A laser head 34 then scans over the powder bed, and a laserbeam from the head is turned on and off to melt the powder in a desiredpattern. The support member 33 then moves down by a small distance(typically of the order of 0.1 mm) to prepare for growth of the nextlayer. After a pause for the melted powder to solidify, the roller 32proceeds to roll another layer of powder over support member 33 inpreparation for sintering. Thus as the process proceeds, a sintered part35 is constructed, supported by unconsolidated powder parts 36. Afterthe part has been completed, it is removed from support member 33 andthe unconsolidated powder 36 is recycled before being returned to thefeed containers 30, 31.

The powder bed system of FIG. 9 can be used to construct the entirefloating rib foot 1, including the web portion 3, flanges 2 andprojections 5. Movement of the laser head 34 and modulation of the laserbeam is determined by a Computer Aided Design (CAD) model of the desiredprofile and layout of the part.

The powder feed fabrication system shown in FIG. 10 can be used to buildup the projections 5 on a previously manufactured floating rib foot.That is, the web portion 3 and flanges 2 have been previouslymanufactured before being mounted in the powder feed fabricationmechanism.

A projection 5 is shown being built up on the underside of one of theflanges 2 in FIG. 10. The powder feed fabrication system comprises amovable head 40 with a laser 41 and an annular channel 42 around thelaser 41. Un-sintered powder flows through the channel 42 into the focusof the laser beam 43. As the powder is deposited, it melts to form abead 44 which becomes consolidated with the existing material.

The powder feed system may be used to grow the projections in series, orin parallel. More specifically, the projections may be grown in parallelby the following sequence:

P(1)L(1), P(2)L(1), . . . P(n)L(1), P(1)L(2), P(2)L(2), . . . P(n)L(2) .. . etc.

or in series by the following sequence:

P(1)L(1), P(1)L(2), . . . P(1)L(m), P(2)L(1), P(2)L(2), . . . P(2)L(m) .. . etc.

where P(X)L(Y) represents the growth of a layer X of a projection Y.

This can be contrasted with the powder bed system which can only growthe projections in parallel.

In contrast to the powder bed system of FIG. 10, the powder feed systemof FIG. 9 directs powder to only the selected parts of the bond region,and fuses the powder as it is delivered. Therefore the powder feedmechanism produces structures that are unsupported by powder, and sosupports (not shown) may need to be built integrally into the part andmachined off later, in particular where the projections have largeoverhanging parts.

The head 40 may be the only moving feature in the process, or the partmay be rotated during fabrication. In other words, the head 40 directspowder to selected parts of the bond region with the part in a firstorientation relative to the head 40; the part is rotated so that itadopts a second orientation relative to the head 40; and the head thendirects material to selected parts of the bond region with the part inthe second orientation. This facilitates the manufacturing of complexshapes without the need for removable supports. For instance overhangingfeatures can be formed by rotating the part between layers in order toalways ensure that the element being built is at no more than 30 degreesfrom the vertical. As the build area is at a temperature significantlybelow the melting point of the material, the area being built will onlyneed to maintain a supportable angle for a brief time after the laserenergy is removed in order for it to solidify enough to become selfsupporting. If the projections are built in a parallel sequence then itis possible to re-orientate the part between each layer to enableunsupported overhanging features to be built.

The laser source of either the powder bed or powder feed systems can bereplaced by another power beam source, such as an electron beam sourcefor directing an electron beam.

Whilst the projections 5 of this embodiment are rotationally symmetricabout their centreline, and the centreline is linear and orientedperpendicularly to the bond surface 6, the projections may take manydifferent forms within the scope of this invention. For example, theprojections need not be rotationally symmetric and the conic sections ofthe base, tip and overhang may by replaced with pyramidal sections. Thecentreline need not be linear but may be curved. The centreline need notbe oriented perpendicularly to the bond surface at the point ofintersection, but instead the centreline may form an oblique angle withthe bond surface at this intersection.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

The invention claimed is:
 1. A joint between a first component and asecond component, wherein: the first component includes an array ofprojections formed extending from a bond surface thereof, theprojections having a plurality of different profiles; and the secondcomponent includes a plurality of laminate plies laid over the bondsurface of the first component, wherein the array of projections areembedded in the second component, each projection has a height, and eachdifferent projection profile has a different height such that theprojections penetrate through differing numbers of the laminate pliesdepending on their height.
 2. The joint according to claim 1, whereineach projection has at least one overhanging edge, and each differentprojection profile has its overhanging edge(s) at a different heightwith respect to the bond surface.
 3. The joint according to claim 1,wherein the projections are arranged such that some adjacent projectionshave different profiles.
 4. The joint according to claim 1, wherein theprojections are arranged such that some adjacent projections have thesame profile.
 5. The joint according to claim 1, wherein the projectionsare arranged such that groups of projections form a repeating profilepattern on the bond surface.
 6. The joint according to claim 1, whereinthe projections are all of similar average width dimension.
 7. The jointaccording to claim 1, wherein each projection is rotationally symmetricabout its centerline.
 8. The joint according to claim 1, wherein eachprojection has a pointed tip.
 9. The joint according to claim 1, whereineach projection has a frustoconical base, a conical tip and an invertedfrustoconical overhang between the base and the tip.
 10. The jointaccording to claim 1, wherein the first component is metallic.
 11. Thejoint according to claim 1, wherein the laminate plies are fibrereinforced composite plies.
 12. The joint according to claim 1, whereineach different projection profile has an overhanging edge at a differentheight with respect to the bond surface.
 13. The joint according toclaim 1, wherein the laminate plies are laminate plies of a compositewing cover.
 14. An aerospace structure, comprising: a fuselage includingthe joint of claim
 1. 15. An aerospace structure, comprising: a wingincluding the joint of claim
 1. 16. The joint of claim 1, wherein: thearray of projections includes a plurality of first projections and aplurality of second projections, the first projections have first andsecond frusto-conical sections arranged back-to-back, the secondprojections have third and fourth frusto-conical sections arrangedback-to-back, and a height of a first area of adjacency between thefirst and second frusto-conical sections above the first component isdifferent than a height of a second area of adjacency between the thirdand fourth frusto-conical sections above the first component.
 17. Thejoint according to claim 1, wherein the joint joins aerospace structuralcomponents.
 18. A joint between a first component and a secondcomponent, wherein: the first component includes an array of projectionsformed extending from a bond surface thereof, the projections having aplurality of different profiles; and the second component includes aplurality of laminate plies laid over the bond surface of the firstcomponent, wherein the array of projections are embedded in the secondcomponent, each projection has an overhanging edge, and each differentprojection profile has its overhanging edge at a different height withrespect to the bond surface.
 19. The joint according to claim 18,wherein each different projection profile has a different height withrespect to the bond surface.
 20. The joint according to claim 18,wherein the projections are arranged such that some adjacent projectionshave different profiles.
 21. The joint according to claim 18, whereinthe projections are arranged such that some adjacent projections havethe same profile.
 22. The joint according to claim 18, wherein theprojections are arranged such that groups of projections form arepeating profile pattern on the bond surface.
 23. The joint accordingto claim 18, wherein the projections are all of similar average widthdimension.
 24. The joint according to claim 18, wherein each projectionis rotationally symmetric about its centerline.
 25. The joint accordingto claim 18, wherein each projection has a pointed tip.
 26. The jointaccording to claim 18, wherein each projection has a frustoconical base,a conical tip and an inverted frustoconical overhang between the baseand the tip.
 27. The joint according to claim 18, wherein the firstcomponent is metallic.
 28. The joint according to claim 18, wherein thelaminate plies are fibre reinforced composite plies.
 29. The jointaccording to claim 18, wherein the laminate plies are laminate plies ofa composite wing cover.
 30. The joint according to claim 18, wherein thejoint joins aerospace structural components.
 31. The joint of claim 18,wherein: the array of projections includes a plurality of firstprojections and a plurality of second projections, the first projectionshave first and second frusto-conical sections arranged back-to-back, thesecond projections have third and fourth frusto-conical sectionsarranged back-to-back, and a height of a first area of adjacency betweenthe first and second frusto-conical sections above the first componentis different than a height of a second area of adjacency between thethird and fourth frusto-conical sections above the first component.